This is extremely dependent on the efficiency of your system and how well it is designed. A 15 SEER heat pump can't compare to a 21 SEER (all else equal). Ther

This is extremely dependent on the efficiency of your system and how well it is designed. A 15 SEER heat pump can't compare to a 21 SEER (all else equal). There are heat pumps that do well below freezing, they're just top of the line and so more expensive.